Encouraging photoprotection is a leading preventative health strategy used by physicians involved in skin care. Although sun avoidance is most desirable, outdoor occupations and lifestyles make total avoidance impossible for many individuals.
Solar UV (Ultraviolet) insults often damage cells, leading to premature photoaging (1-8) and carcinogenesis (9-17). Currently, there are 2 main method used to protect mammalian skin against UV damage.
In the first method, a composition of matter that absorbs ultraviolet radiation, also called a sunscreen, is applied to the skin of a subject. Therefore, only a relatively small quantity of UV radiation reaches the skin. However, the substances contained in the composition of matter are degraded by the UV radiation and other environmental factors. Accordingly, the protection provided by a single application of the composition of matter lasts only for a relatively short amount of time, and there is a need to frequently reapply the composition of matter on the skin of the subject. Also, some authors suggest that at least some of the substances used to protect against UV damage might themselves be carcinogenic.
Furthermore, the UV absorbing substances are often mixed with hydrophobic substances to better penetrate the skin. They therefore produce an often undesirable “oily” texture onto the skin. This disadvantage may be strong enough that some subjects refuse to use the composition of matters.
UV radiation (UVR) that reaches the Earth's surface can be divided into UV-B (290-320 nm) and UV-A (320-400 nm). UV-A can be further subdivided into UV-A I, or far UV-A (340-400 nm), and UV-A II, or near UV-A (320-340 nm). The sun protection factor (SPF) is defined as the dose of UVR required to produce 1 minimal erythema dose (MED) on protected skin after the application of 2 mg/cm2 of product divided by the UVR required to produce 1 MED on unprotected skin. A water-resistant product maintains the SPF level after 40 minutes of water immersion, whereas a very water-resistant (also known as waterproof) product maintains the SPF level after 80 minutes of water immersion. A broad-spectrum or full-spectrum sunscreen provides both UV-B and UV-A protection, ideally through the entire UV-A I and UV-A II range. Erythema, the key measurement in the SPF assay, is a relatively crude biologic endpoint. A comparison of a SPF 15 sunscreen versus a SPF 30 sunscreen showed subclinical damage (sunburn cell formation) in the former without visible erythema. The SPF 30 product provided significantly greater protection. Other forms of subclinical damage may occur with a SPF 15 formulation.
Although UV-A protection may be less than desirable with all sunscreen products, the UV-A protection is better with a higher SPF, particularly in the UV-A II (320-340 nm) or shorter UV-A range. Although sunscreens provide excellent UV-B protection, they lack in UV-A protection, particularly UV-A I. No consensus exists about the best method for measuring UV-A protection. A variety of methods have been proposed. In vivo methods have been developed on the basis of direct UV-A erythema, persistent pigment darkening, and photosensitization with psoralens. At best, each method has its limitations and indications for a particular clinical situation or skin type. An in vitro method relying on transmittance through a thin substrate, such as thin film, may be more practical and is currently used in Europe. If protection from UVR into the UV-A I range is desired, the formula should contain either avobenzone or an inorganic particulate sunscreen as an active ingredient.
Sunscreens alone may also provide insufficient protection from UVR. Sunscreens function best to prevent sunburn from UV-B radiation. They provide more limited protection from UV-A radiation. Sole dependence on sunscreens can have the unwanted effect of increasing outdoor exposure times, particularly in those individuals who burn easily and tan poorly. Sun avoidance remains the most desirable form of sun protection.
Typically, sunscreen should be applied 15-30 minutes prior to sun exposure to allow sufficient time for a protective film to develop. Sunscreen should be reapplied after prolonged swimming or vigorous activity. Sunscreen needs to be applied in relatively large quantities. As much as 1 oz may be needed to cover the entire body. Particular attention needs to be paid to the back of the neck, the ears, and the areas of the scalp with thin hair. All these limitations often result in improper use of sunscreens, and consequently to sub-optimal protection from damaging radiations.
In the second method, the skin is irradiated with UV radiation under controlled conditions to stimulate the natural production of UV absorbing skin pigments. A disadvantage of this method is that while some damage is prevented when a solar UV exposition is performed onto the skin thereafter, the preventive treatment still causes some potential UV skin damage.
The present document makes reference to a number of documents, the contents of which is hereby incorporated by reference in their entirety.
Against this background, there exists a need in the industry to provide novel methods for protecting mammalian skin against upcoming photodamage.
An object of the present invention is therefore to provide an improved method for protecting mammalian skin against upcoming photodamage.